1. Field of the Invention
The present invention relates to a printing mask and a printing method for printing a solder paste on lands on a printed circuit board for mounting various electronic parts thereon, and more particularly to a surface-mounted structural assembly having a printed circuit board with various electronic parts mounted thereon by soldering and a method of manufacturing such a surface-mounted structural assembly.
2. Description of the Related Art
It has heretofore been the customary practice to use soldering to mount electronic parts on a printed circuit board (hereinafter referred to as “PCB”). One example of a process of mounting electronic parts on both surfaces of a PCB by reflow soldering will be described below with reference to FIG. 1 of the accompanying drawings.
In step 101, a solder paste is printed on lands on a surface of a PCB using a printing mask having holes which are defined complementarily to the lands. In step 102, surface-mount components including chip parts, QFPs (Quad Flat Packages), SOPs (Small Outline Packages), etc. are placed on the printed solder paste. Then, in step 103, the PCB with the surface-mount components placed thereon is passed through a high-temperature reflow furnace to melt the solder paste, thus soldering the leads of the surface-mount components to the lands on the PCB. After the surface-mount components have thus been mounted on one surface of the PCB, the PCB is inverted in step 104, so that the other surface of the PCB faces upwardly.
In steps 105, 106, a solder paste is applied to and surface-mount components are placed on the other surface of the PCB in the same manner as with steps 101, 102. Thereafter, leads of electronic parts (hereinafter referred to as “through-hole parts”) to be mounted in through holes in the PCB are inserted into the through holes in the PCB in step 107, thus placing the through-hole parts on the PCB. The PCB is then passed through the reflow furnace to melt the solder paste, thus soldering the through-hole parts to the PCB in step 108 in the same manner as with step 103.
Finally, any components which cannot withstand the high temperature in the reflow furnace are manually soldered to the PCB in step 109, thus completing the mounting of necessary electronic parts on the PCB.
In the conventional process of mounting electronic parts on a PCB, there has generally been used an Sn—Pb solder as the solder paste. However, since the Sn—Pb solder contains Pb that is a toxic heavy metal, it tends to adversely affect the environment if electronic devices including those PCBs are not properly processed after use. For this reason, there has recently been a demand for the use of Pb-free solder materials on PCBs to prevent environmental pollution.
Sn—Ag solder has widely been known as Pb-free solder. Since the properties of Ag are stable, the Sn—Ag solder is as reliable as the Sn—Pb solder when used to mount electronic parts on PCBs. One problem of the Sn—Ag solder is that the melting point of the Sn—Ag solder is about 220° C. that is higher than the melting point of the Sn—Pb solder which is about 183° C. Therefore, it is difficult to directly use surface-mounting facilities and processes that have used the Sn—Pb solder to solder electronic parts with the Sn—Ag solder. Specifically, because general electronic parts have a heat resistance temperature of about 230° C., if the Sn—Ag solder is melted in the reflow furnace to solder the electronic parts, then the temperature of the electronic parts may possibly reach 240° C. or higher. Consequently, if electronic parts are to be mounted on PCBs with the Sn—Ag solder, then it is necessary to increase the heat resistance temperature of those electronic parts.
Another type of Pb-free solder is Sn—Zn solder. Since the Sn—Zn solder has a melting point of about 197° C., the conventional surface-mounting facilities and electronic parts can directly be used if the Sn—Zn solder is applied to solder the electronic parts.
However, the Sn—Zn solder is disadvantageous as compared with the Sn—Pb solder in that Zn is easily oxidizable and makes wetting poor. If electronic parts are mounted on PCBs with the Sn—Zn solder using the conventional surface-mounting facilities and processes, the Sn—Zn solder is not as reliable as the Sn—Pb solder.
The above paste printing process will be described below with reference to FIGS. 1 and 2A through 2C of the accompanying drawings. An interconnection pattern of copper foil has been covered with an insulating resist layer on a PCB, and lands are provided on the PCB by removing the resist layer from portions of the interconnection pattern. The resist layer is omitted from illustration in FIGS. 2A, 2B, and 2C.
As shown in FIG. 2A, printing mask 111 is positioned on PCB 112 with through holes 116 defined therein being aligned with respective lands 115 on PCB 112. Then, a given amount of solder paste 113 is placed on printing mask 111 on PCB 112. As shown in FIG. 2B, solder paste 113 is turned and moved over printing mask 111 from one end to the other by squeegee 118.
As solder paste 113 is turned and moved over printing mask 111, it is pushed into through holes 116 by squeegee 118 and filled in through holes 116. As shown in FIG. 2C, when printing mask 111 is peeled off PCB 112, a given layer of solder paste 113 is printed on each land 115 on PCB 112. Now, the solder paste printing process is completed.
Use of the Sn—Zn solder in the process of printing the solder paste on the PCB and performing reflow soldering poses the following problems: When the solder paste is filled in the through holes in the PCB and through-hole parts are mounted thereon, since the Sn—Zn solder has poor wettability, if the Sn—Zn solder is applied in the same amount as the conventional Sn—Pb, then solder fillets around the leads of the through-hole parts may be small or no solder fillets may be formed around the leads of the through-hole parts on the surface of the PCB opposite to the surface of the PCB that is printed with the solder paste. Therefore, the through-hole parts may not be well connected to the lands.
In view of the above problems, it has been proposed to increase the opening areas of the through holes in the printing mask in order to increase the amount of solder paste filled in the through holes and the amount of flux for thereby increasing wettability with the leads and the through holes. However, because the through holes for insertion of the leads of the through-hole parts are defined in adjacent positions in the PCB, if the printing mask with the increased opening areas of the through holes is used, then the applied solder paste is liable to extend between the leads passing through the adjacent through holes.
Therefore, if the printing mask with the increased opening areas of the through holes is used, then a solder bridge may be formed between the leads of the through-hole parts mounted on the PCB, and the solder paste flowing from between the leads may be turned into a number of balls (solder balls).